For discrimination of biologically-relevant microparticles such as cells, microbes, and liposomes, an optical detection method using flow cytometry (flow cytometer) is used. The flow cytometry is an analytical technique to irradiate particles flowing in line within a flow path with laser light of a specific wavelength and detect fluorescence or scattered light emitted from each of the particles, to discriminate the particles from one another.
Meanwhile, in the flow cytometry, an oscillation wavelength of a laser fluctuates in the order of several tens of μs due to the influences of the flow path, return light from the particles, and the like. This causes a change in the amount of light output from the laser, and this change is detected as noise. Such noise is generally called “mode hopping noise” and becomes a cause of variations in detection data or of a reduction in reliability.
The flow cytometry is provided with a laser output adjustment system (auto power control: APC) in order to prevent a laser output level from fluctuating due to the influence of temperature or the like. However, feedback control by the APC is performed in the order of several ms and cannot cope with mode hopping noise that occurs in the order of several tens of μs.
In the past, the use of high frequency superimposition has been proposed as a method of reducing the mode hopping noise and making the laser output stable (see Patent Document 1). For example, in a sample analyzer described in Patent Document 1, a high-frequency superimposition circuit that superimposes a high-frequency component on a direct current supplied to a laser diode is provided, and the amplitude of the high-frequency wave superimposed on the direct current is controlled in accordance with the magnitude of the direct current supplied to the laser diode.
Patent Document 1: Japanese Patent Application Laid-open No. 2009-53020